The present invention relates to the field of sensors for measuring a gas concentration or ion concentration.
Sensors with field effect transistors (FETs) that have an ion-sensitive layer used as a gate are used to measure ion concentrations, with the potential of the layer depending on the ion concentration of a surrounding fluid or gas. For example, U.S. Pat. No. 5,911,873 shows such an ion-sensitive FET (ISFET). In addition, sensors with FETs are known for measuring gas concentrations, for example from U.S. Pat. No. 4,411,741, which have a gas-sensitive layer used as a gate, whose work function depends on the surrounding gas concentration.
Such sensors are generally produced from a drain and a source in a semiconductor substrate by counterdoping, and an insulating layer is grown or deposited on the substrate between the source and the drain. An ion-sensitive layer can be applied directly to this insulating layer. A gas-sensitive layer called a suspended gate FET (SGFET) can be applied at a certain distance. Alternatively, a gate can be applied to the insulator that is controlled capacitively by a gas-sensitive gate applied at a certain distance. This type of sensor is referred to as a capacitive-controlled FET (CCFET), and is described for example in German patent document DE 43 33 875 C2.
One disadvantage of this arrangement is that because of the ever-present surface conductivity, the potential is drawn over the FET after a certain period of time to the potential that is applied to the gas-sensitive gate, which causes a drift of the drain source current. To prevent this, a conductive ring, also called a guard ring and connected to a definite potential is applied conventionally to the FET. With such an arrangement, the channel area of the FET, because of the surface conductivity of the area between the guard ring and the channel area, assumes the potential of the guard ring after a certain period of time. The distance of the guard ring from the channel area of the FET and the conductivity of the surface define the time until the channel area assumes the guard ring potential, so that the minimum possible concentration change per unit time that a gas signal to be detected must have in order to be recorded is established. This distance determines the size, and as a result also the manufacturing cost of such a sensor.
Therefore, there is a need for a sensor for measuring an ion concentration or a gas concentration, which is economical, compact, and ensures measurement accuracy for the change in concentration with time.